I. Field of the Invention
The present invention relates to data communication. More particularly, the present invention relates to a novel and improved method and apparatus for providing power control in a communication system.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Other multiple access communication system techniques, such as time division multiple access (TDMA) and frequency division multiple access (FDMA) are known in the art. However, the spread spectrum modulation techniques of CDMA have significant advantages over other modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,xe2x80x9d assigned to the assignee of the present invention and is incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d also assigned to the assignee of the present invention and is incorporated by reference herein. Furthermore, the CDMA system can be designed to conform to the xe2x80x9cTIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systemxe2x80x9d, hereinafter referred to as the IS-95 standard or TIA/EIA/IS-95.
CDMA, by its inherent nature of being a wideband signal, offers a form of frequency diversity by spreading the signal energy over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth. Space or path diversity is obtained by providing multiple signal paths through simultaneous links to a mobile user or remote station through two or more base stations. Furthermore, path diversity may be obtained by exploiting the multipath environment through spread spectrum processing by allowing signals arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501 entitled xe2x80x9cMETHOD AND SYSTEM FOR PROVIDING A SOFT HANDOFF IN COMMUNICATIONS IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d and U.S. Pat. No. 5,109,390 entitled xe2x80x9cDIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d both assigned to the assignee of the present invention and incorporated by reference herein.
The reverse link refers to a transmission from a remote station to a base station. On the reverse link, each transmitting remote station acts as an interference to other remote stations in the network. The reverse link capacity is limited by the total interference due to transmissions from other remote stations. The CDMA system increases the reverse link capacity by transmitting fewer bits, thereby using less power and reducing interference when the user is not speaking.
To minimize interference and maximize the reverse link capacity, the transmit power of each remote station is controlled by three reverse link power control loops. The first power control loop adjusts the transmit power of the remote station by setting the transmit power inversely proportional to the power of the received forward link signal. In an IS-95 system, the transmit power of the remote station is given by Pout=xe2x88x9273xe2x88x92Pin where Pin is the power received by the remote station given in dBm, Pout is the transmit power of the remote station given in dBm, and xe2x88x9273 is a constant. This power control loop is also referred to as the open loop.
The second power control loop adjusts the transmit power of the remote station such that the signal quality, as measured by the energy-per-bit-to-noise-plus-interference ratio Eb/Io, of the reverse link signal received at the base station is maintained at a predetermined level. This level is referred to as the Eb/Io set point. The base station measures the Eb/Io of the reverse link signal received at the base station and transmits a reverse power control bit to the remote station on the forward traffic channel in response to the measured Eb/Io. For IS-95 communication systems, the reverse power control bits are sent 16 times per 20 msec frame, or one power control bit per power control group, for an effective rate of 800 bps. The forward traffic channel carries the reverse power control bits along with the data from the base station to the remote station. This second loop is also referred to as the closed loop.
The CDMA communication system typically transmits packets of data as discrete data frames. Thus, the desired level of performance is typically measured by the frame-error-rate (FER). The third power control loop adjusts the Eb/Io set point such that the desired level of performance, as measured by the FER, is maintained. The required Eb/Io to maintain a given FER depends upon the propagation conditions. This third loop is also referred to as the outer loop. The power control mechanism for the reverse link is disclosed in detail in U.S. Pat. No. 5,056,109, entitled xe2x80x9cMETHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEMxe2x80x9d, assigned to the assignee of the present invention and incorporated by reference herein.
The forward link refers to a transmission from the base station to the remote station. On the forward link, the transmit power of the base station is controlled for several reasons. A high transmit power from the base station can cause excessive interference with other signals received at the remote stations. Alternatively, if the transmit power of the base station is too low, the remote stations can receive erroneous data transmissions. Terrestrial channel fading and other known factors can affect the quality of the forward link signal as received by the remote station. As a result, each base station attempts to adjust its transmit power to maintain the desired level of performance at the remote station.
Power control on the forward link is especially important for data transmissions. Data transmission is typically asymmetric with the amount of data transmitted on the forward link being greater than on the reverse link. With an effective power control mechanism on the forward link, wherein the transmit power is controlled to maintain the desired level of performance, the overall forward link capacity can be improved.
A method and apparatus for controlling the forward link transmit power is disclosed in U.S. Pat. No. 6,035,209 which is a continued prosecution application of U.S. patent application Ser. No. 08/414,633, now abandoned, entitled xe2x80x9cMETHOD AND APPARATUS FOR PERFORMING FAST FORWARD POWER CONTROL IN A MOBILE COMMUNICATION SYSTEMxe2x80x9d, filed Mar. 31, 1995, assigned to the assignee of the present invention and incorporated by reference herein. In the method disclosed in U.S. Pat. No. 6,035,209, the remote station transmits an error-indicator-bit (EIB) message to the base station when a transmitted frame of data is received in error. The EIB can be either a bit contained in the reverse traffic channel frame or a separate message sent on the reverse traffic channel. In response to the EIB message, the base station increases or decreases its transmit power to the remote station.
One of the disadvantages of this method is the long response time. The processing delay encompasses the time interval from the time the base station transmits the frame with inadequate power to the time the base station adjusts its transmit power in response to the error message from the remote station. This processing delay includes the time it takes for (1) the base station to transmit the data frame with inadequate power, (2) the remote station to receive the data frame, (3) the remote station to detect the frame error (e.g. a frame erasure), (4) the remote station to transmit the error message to the base station, and (5) the base station to receive the error message and appropriately adjust its transmit power. The forward traffic channel frame must be received, demodulated, and decoded before the EIB message is generated. Then the reverse traffic channel frame carrying the EIB message must be generated, encoded, transmitted, decoded, and processed before the bit can be used to adjust the transmit power of the forward traffic channel.
Typically, the desired level of performance is one percent FER. Therefore, on the average, the remote station transmits one error message indicative of a frame error every 100 frames. In accordance with the IS-95 standard, each frame is 20 msec long. This type of EIB based power control works well to adjust the forward link transmit power to handle shadowing conditions, but due to its slow speed is ineffective in fading except in the slowest fading conditions.
A second method for controlling the forward link transmit power utilizes the Eb/Io of the received signal at the remote station. Since the FER is dependent on the Eb/Io of the received signal, a power control mechanism can be designed to maintain the Eb/Io at the desired level. This design encounters difficulty if data is transmitted on the forward link at variable rates. On the forward link, the transmit power is adjusted depending on the data rate of the data frame. At lower data rates, each data bit is transmitted over a longer time period by repeating the modulation symbol as described in TIA/EIA/IS-95. The energy-per-bit Eb is the accumulation of the received power over one bit time period and is obtained by accumulating the energy in each modulation symbol. For an equivalent amount of Eb, each data bit can be transmitted at proportionally less transmit power at the lower data rates. Typically, the remote station does not know the transmission rate a priori and cannot compute the received energy-per-bit Eb until the entire data frame has been demodulated, decoded, and the data rate of the data frame determined. Thus, the delay of this method is about that described in the aforementioned U.S. Pat. No. 6,035,209 and the rate is one power control message per frame. This is in contrast with the reverse link power control mechanism described above wherein one power control message (bit) is sent sixteen times per frame as specified by TIA/EIA/IS-95.
Other methods and apparatus for performing fast forward link power control are described in the aforementioned U.S. Pat. No. 6,035,209 and U.S. patent application Ser. No. 08/559,386, entitled xe2x80x9cMETHOD AND APPARATUS FOR PERFORMING FAST FORWARD POWER CONTROL IN A MOBILE COMMUNICATION SYSTEMxe2x80x9d, filed Nov. 15, 1995, U.S. Pat. No. 5,903,554, entitled xe2x80x9cMETHOD AND APPARATUS FOR MEASURING LINK QUALITY IN A SPREAD SPECTRUM COMMUNICATION SYSTEMxe2x80x9d, filed Sep. 27, 1996, U.S. Pat. No. 5,893,035 entitled xe2x80x9cMETHOD AND APPARATUS FOR PERFORMING DISTRIBUTED FORWARD POWER CONTROLxe2x80x9d, filed Sep. 16, 1996, and U.S. Pat. No. 6,075,974 entitled xe2x80x9cADJUSTMENT OF POWER CONTROL THRESHOLD/MEASUREMENTS BY ANTICIPATING POWER CONTROL COMMANDS THAT HAVE NOT BEEN EXECUTEDxe2x80x9d, filed Nov. 20, 1996, all assigned to the assignee of the present invention and incorporated by reference herein.
For IS-95 systems, the fundamental difference between the forward and reverse links is that the transmission rate does not need to be known on the reverse link. As described in the aforementioned U.S. Pat. No. 5,5056,109, at lower rates, the remote station does not transmit continuously. When the remote station is transmitting, the remote station transmits at the same power level using the same waveform structure regardless of the transmission rate. The base station determines the value of a power control bit based on the Eb/Io measurement of the received reverse link signal and sends this power control bit to the remote station 16 times per frame. The base station can ignore power control bits corresponding to times when the remote station is not transmitting. This permits fast reverse link power control. However, the effective power control rate varies with the transmission rate. For TIA/EIA/IS-95, the rate is 800 bps for full rate frames and 100 bps for xe2x85x9 rate frames.
An alternative reverse link architecture is described in the U.S. Pat. No. 5,930,230, entitled xe2x80x9cHIGH DATA RATE CDMA WIRELESS COMMUNICATION SYSTEMxe2x80x9d, filed May, 28, 1996, assigned to the assignee of the present invention and incorporated by reference herein. In accordance with U.S. Pat. No. 5,930,230, an auxiliary pilot is introduced on the reverse link. The pilot level is independent of the transmission rate on the reverse link. This permits the base station to measure the pilot level and to send the reverse power control bit to the remote station at a constant rate.
These various methods for providing power control of the forward and reverse links in the prior art utilize one-bit power control command to direct the source unit (remote station or base station) to increase or decrease its transmit power depending on the measured Eb/Io of the received signal at the receiving unit (base station or remote station). The one-bit command minimizes the number of bit which are transmitted for the power control function, thus minimizing the overhead required by the system and reserving more resources for data transmission. However, the one-bit command inherently causes toggling (or limit cycling) of the power control since the transmit power is either increased or decreased at each power control group depending on the received value of the power control bit. Furthermore, due to processing delay, the transmit power can be adjusted in the wrong direction for several power control groups before correction is made, thereby magnifying the affect of limit cycling. The limit cycling can reduce the efficiency and performance of the communication system. A method is needed to control the transmit power of the source unit by utilizing the minimal number of bits while reducing or eliminating the limit cycling of the transmit power which is inherent with a one-bit power control mechanism.
The present invention is a novel and improved method and apparatus for providing power control in a communication system which utilizes a ternary signaling scheme. The present invention improves the performance of the communication system by reducing or eliminating limit cycling which is inherent in a binary signaling scheme. In the exemplary embodiment, power control values (each having one of three possible values) are punctured onto the data to improve the response time of the power control loop and allow for dynamic adjustment of the transmit power. The power control mechanism of the present invention can be utilized on the forward link and/or the reverse link. However, for simplicity, the present invention is described in the context of the reverse link power control.
It is an object of the present invention to provide a ternary power control signaling scheme. In the exemplary ternary signaling scheme, a power up command is represented by a positive value (e.g., +1), a power down command is represented by a negative value (e.g., xe2x88x921), and a do nothing command is represented by a zero. The ternary signaling scheme minimizes the number of bits which is allocated for the power control function, thereby reserving more resources for data transmission.
It is another object of the present invention to improve the performance of the communication system by reducing or eliminating limit cycling in the power control loop. In the exemplary embodiment, a power control value comprises a power up, a power down, or a do nothing command. In the exemplary embodiment, if the quality of the received signal (e.g., as measured by the energy-per-bit-to-noise-plus-interference ratio Eb/Io) is within a predetermined range, the base station transmits the do nothing command. The do nothing command minimizes limit cycling which is inherent in a binary signaling scheme. The do nothing command also minimizing variations in the transmit power of the remote station due to Eb/Io measurement uncertainty of the received signal at the base station.
It is yet another object of the present invention to improve the response time of the power control loop. In the exemplary embodiment, the power control values are transmitted to the remote station without encoding. Furthermore, the power control values are punctured onto the encoded data. At the remote station, the power control values can be demodulated and detected rapidly without having to endure the long decoding process. The quick response time improves the performance of the power control loop and can result in improved performance and increased capacity of the communication system.
It is yet another object of the present invention to provide a power control mechanism which supports handoff. The remote station can be in soft handoff with multiple base stations and receive identical or non-identical power control values from the base stations. At the remote station, the transmitted power control values are received, demodulated, and filtered. Identical power control values from multiple base stations or multiple signal paths are combined to produce an improved measurement of the power control value. Each independent power control value is compared against a set of thresholds to produce the corresponding received power control value. The received power control values from all base stations in communication with the remote station are then logically combined such that the remote station reduces its transmit power if any base station sends a power down command, does nothing if no base stations send a power down command and at least one base station sends a do nothing command, and increases its transmit power if all base stations send power up command.
It is yet another object of the present invention to provide for a reliable power control mechanism. The reverse power control bits which are deemed unreliable may be omitted from use in the power control loop, e.g., by maintaining the transmit power.
Although the present invention has been described for the reverse link power control, the inventive concept can be fully adapted for use in forward link power control.